This invention relates to portable apparatus used for geophysical exploration for buried objects such as mines, relics and minerals, which will be referred to by the generic terms targets and objects.
The prior art detectors utilize either a variable frequency tone or a single frequency variable volume tone as an acoustical presentation of the absence or presence, and proximity of a buried object. Both of these acoustical presentation systems are subject to several problems. The single frequency systems commonly use a tone such as 1000 Hz; this tone may be hazardous to hearing if operated for more than one-half hour per day. Furthermore, use of a single frequency tone presents a perceptual loudness adaptation problem. When the operator has listened to the tone for a few minutes, the tone is perceived by him as being fainter even though the level has remained constant, resulting in confusion as to whether the tone has changed or not.
The use of a variable frequency tone, likewise, has its disadvantages. While the fact that the sound level does not increase when approaching a target reduces the hearing hazard, the constant drone is still irritating. In addition, many people cannot discern if a tone is increasing or decreasing in frequency.
The prior art detector systems also present the user with a problem in terms of operation. The handheld search head utilized to detect the mines or other buried objects must be swept back and forth in an arc at a specific constant rate, usually 3 ft/sec, due to processing requirements; however, there has existed up until now no effective way for the operator to pace his sweep or hold it relatively constant for any significant period of time.
A further disadvantage of prior art systems was that the manner in which signals were processed made operation in certain soil conditions difficult or impossible with false alarms and missed or lost targets a frequent occurrence. A detector with preset threshold parameters will generate false alarms when noise levels are high, as is common with wet soil conditions, since the noise signal will often exceed the reference point by more than the amount required to trigger the detection signal; in contrast, with dry soil only a small deviation from the reference point may be indicative of a target, such that threshold parameters preset to levels in a range necessary to prevent false alarms will miss small objects which might not present a signal deviation high enough to be detected. On the other hand, a detector that uses the preceding sweep level as the background reference point with a preset deviation value for detection of targets cannot pinpoint a target since it will lose the signal after detection due to the reference signal being automatically updated toward the target's level, thereby bringing it within a non-triggering deviation level. A system that uses the latter method for detection and the former system for pinpointing can lose the target if the preset threshold level is above the level developed from the use of the actual background readings.